© ISO ISO 3664:1999(E)

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Viewing conditions - for Graphic Technologyand Photography

(Revision of ISO 3664 - 1975, Photography -Illumination conditions for viewing colourtransparencies and their reproductions)

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Contents

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Normative references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

4 Viewing conditions requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

5 Test methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Annexes

A Summary of ISO viewing conditions specified in ISO 3664 . . . . . . . . . . 12

B Experimental data leading to selection of metameric indices and referenceilluminant for this International Standard . . . . . . . . . . . . . . . . . . . . . . . . 13

C Guidelines for judging and exhibiting photographs . . . . . . . . . . . . . . . . 19

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Foreword

ISO (the International Organization for Standardization) is a worldwide federationof national standards bodies (ISO member bodies). The work of preparingInternational Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has beenestablished has the right to be represented on that committee. Internationalorganizations, governmental and non-governmental, in liaison with ISO, also takepart in the work. ISO collaborates closely with the International ElectrotechnicalCommission (IEC) on all matters of electrotechnical standardization.

Draft International Standards adopted by the technical committees are circulated tothe member bodies for voting. Publication as an International Standard requiresapproval by at least 75% of the member bodies casting a vote.

International Standard ISO 3664 was revised by a joint working group, composed ofmembers of ISO/TC 6, Paper, board and pulps; ISO/TC 42, Photography andISO/TC 130, Graphic technology. Annexes A to C of this International Standard are for information only.

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Introduction

While colour and density measurements play important roles in the control of colourreproduction, they cannot replace the human observer for final assessment of thequality of complex images. Colour reflection artwork, photographic transparencies,photographic prints, and photomechanical reproductions such as on-press and off-press proofs, or press sheets, are commonly evaluated for their image and colourquality, or compared critically with one another for fidelity of colour matching. Paper and other substrates contribute to the colour appearance and controlling thecolour of these is equally critical. However, it should be noted that the paperindustry has its own set of International Standards for unprinted paper which differin illumination conditions from those recommended in this International Standard.

There is no doubt that the best viewing condition for the visual assessment of colouris that in which the product will be finally seen. Where this is known, and it ispractical to do so, the various people in the production chain may sensibly agree touse this viewing condition for all evaluation and comparison. However, it isimportant that this be properly agreed upon in advance and that it be specified thatsuch a viewing condition is NOT ISO-defined.

Unfortunately, such agreement is often not practical. Even if a particular end usecondition is known, it may be impractical to provide everybody in the productionchain with sufficiently consistent viewing apparatus. Since deficiencies in lightsources and viewing conditions, and inconsistencies between colour viewingfacilities, can distort the colour appearance of substrates, reproductions and artwork,they are likely to cause miscommunication about colour reproduction andprocessing. This International Standard provides specifications for illumination andviewing conditions, that when properly implemented, will reduce errors andmisunderstandings caused by such deficiencies and inconsistencies. This revisionof the 1974 version of the International Standard meets the current needs of theGraphic Technology and Photographic industries and minimizes differencesbetween viewing equipment. It should be noted that this revision contains multiplespecifications, each of which is appropriate to specific requirements. Users shouldensure that they employ the specification which is appropriate to their application.

The illumination used to view colour photographic prints, photomechanicalreproductions, and transparencies needs to provide adequate amounts of radiantpower from all parts of the ultraviolet and visible spectrum to avoid distorting theirappearance from that observed under commonly used sources of illumination suchas daylight. The ultraviolet content is important where fluorescent samples, whichare excited in this region, are encountered; a phenomenon associated with many ofthe paper substrates on which images are reproduced as well as with some of thedyes and pigments themselves.

To ensure consistency with the 1974 International Standard, as well as the majorityof equipment in current use, the reference spectral power distribution specified inthis International Standard is CIE Illuminant D50. Many of the reasons for theselection of illuminant D50 in 1974, as opposed to any other CIE daylight illuminant,

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are equally applicable today. Much consideration was given to changing thereference illuminant to be CIE F8, a 5000 Kelvin illuminant more typical offluorescent lamps. However, it was felt that this would provide only a minimalconformance advantage (as shown in annex B) and the actual goal is for theillumination to simulate natural daylight. Because it is very difficult to produce artificial sources of illumination whichclosely match the spectral power distribution of daylight, it is important that thetolerances specified within this International Standard provide a compromisebetween that required for lamp manufacturing purposes and that for consistentviewing. In this International Standard three constraints which define the colour ofthe light falling on the viewing plane apply, one directly and two indirectly, and allthree must be met simultaneously if a viewing apparatus is to be in compliance.

The chromaticity, which defines directly the colour of the illumination at theviewing surface, is specified as that for illuminant D50 and the tolerance by a circlein the CIE 1976 Uniform Chromaticity Scale (UCS) diagram having a specifiedradius around that value. To establish the compliance of the spectral powerdistribution of the illumination to that of illuminant D50 the methods defined in CIEPublications No. 13.3 and No. 51 are both specified. One defines the colourrendering quality of a lamp; the other its ability to correctly predict metamers. Bothrequirements are important to the Graphic Technology and Photographic industries.

Because CIE Publication No. 51 does not currently address illuminant D50,additional virtual metamers for this illuminant, for both visible and ultravioletevaluation, were calculated and are defined in this International Standard. Theywere derived from those published in CIE Publication No. 51 and are equivalent tothem. Also, based on experimental work described in annex B, a practical toleranceof acceptability has been defined, alongside a Colour Rendering Index requirement.

The perceived tonal scale and colours of a print or transparency can be significantlyinfluenced by the chromaticity and luminance of other objects and surfaces in thefield of view. For this reason, ambient conditions, which may affect the state ofvisual adaptation, need to be designed to avoid any significant effects on theperception of colour and tone and immediate surround conditions need to bespecified also. Such specifications are provided in this International Standard.

Experience in the industries covered by this International Standard has revealed theneed for two levels of illumination; a high level for critical evaluation andcomparison, and a lower level for appraising the tone scale of an individual imageunder illumination levels similar to those under which it will be finally viewed. ThisInternational Standard provides these two levels of illumination.

The higher level is essential to graphic technology where comparison is being made;such as between original artwork and proof, or to evaluate small colour differencesbetween proof and press sheet in order to control a printing operation. It is effectivein these situations because it enhances the visibility of any differences. The highlevel of illumination is also appropriate in photography when comparing two, ormore, transparencies or when critically evaluating a single image to assess thedarkest tones that can be printed. Since, despite adaptation, the level of illumination has quite a significant effect onthe appearance of an image, the lower level is required in order to appraise theimage at a level more similar to that in which it will be finally viewed. Although itis recognised that quite a wide range of illumination levels may be encountered inpractical viewing situations, the lower level chosen is considered to be fairlyrepresentative of the range encountered. For this reason it is applicable to aestheticappraisal, including the conditions for routine inspection of prints.

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The viewing of transparencies is specified both for direct viewing and by projection,and additional conditions where transparencies are to be compared to a print are alsospecified. A particular surround is specified for transparencies which recognises theway that a transparency should be viewed for optimum visibility of the dark tones,but acknowledges that practical viewing equipment is likely to have ambientconditions that introduce some viewing flare. The combination of surround andflare produce an appearance that is fairly representative of how the transparency willlook in a typically lighted room.

Small transparencies are commonly evaluated in graphic technology by directviewing. When it is necessary to view transparencies directly, they should beviewed according to the conditions specified for that situation. However, for somepurposes, smaller transparencies are not viewed directly because the viewingdistance for correct perspective and perception of detail is too small for visualcomfort. Furthermore, when small transparencies are reproduced for publication orother purposes, they are usually enlarged. To ease comparison, it is helpful toenlarge the transparency image when comparing it to the print. For these reasons, aviewing condition may be required which provides a magnified image when viewedat an appropriate distance.

Colour monitors are increasingly being used to display and view digital images ingraphic technology and photography. In order to ensure consistency of assessmentin this situation it is important that the viewing conditions in which the monitors areplaced are reasonably well specified. However, it should be noted that adherence tothese specifications does not ensure that the monitor will match the hardcopywithout provision of a defined colour transformation to the displayed image, or useof proper colour management. This aspect of matching is beyond the scope of thisInternational Standard. In practice, even with high quality colour management, anaccurate match is difficult to achieve because the luminance levels generally differsignificantly between hardcopy (print or transparency) and softcopy (monitor).

Thus, it should be noted that the specifications for images viewed on colourmonitors, provided in this International Standard, are for images viewedindependently of any form of hardcopy; conditions for direct comparisons betweenhardcopy and softcopy (even where a suitable colour transformation has beenapplied) are beyond the scope of this International Standard which can be seen asbeing primarily relevant where successive viewing of hardcopy and softcopy takesplace. ISO 12646, Graphic Technology - Colour proofing using a colour display,currently at Working Draft level in TC 130, is being produced to provide moredetailed recommendations where direct comparison is required. In general it may bestated that for such comparisons it is desirable to view the colour monitor under thelower levels of ambient illumination specified in this International Standard andwith the maximum level of luminance achievable, and the hardcopy sample at thelower levels of illumination specified for printed matter in this InternationalStandard (and their equivalent for transparencies). However, it should be noted thatthis will, in turn, affect the perceived tone and colourfulness of the hardcopy.

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Viewing conditions - For graphic technology and photography

1 Scope

This International Standard specifies viewing conditionsfor images on both reflective and transmissive media, suchas prints (both photographic and photomechanical) andtransparencies, as well as images displayed in isolation oncolour monitors. Specifically, it shall be used for:

- critical comparison between transparencies, reflectionphotographic or photomechanical prints and/or otherobjects or images;

- appraisal of the tone reproduction and colourfulness of

prints and transparencies at illumination levels similarto those for practical use, including routine inspection;

- critical appraisal of transparencies which are viewed byprojection, for comparison with prints, objects, or otherreproductions;

- appraisal of images on colour monitors which are not

viewed in comparison to any form of hardcopy. This International Standard is not applicable to unprintedpapers.

2 Normative references

The following standards contain provisions which, throughreference in this text, constitute provisions of thisInternational Standard. At the time of publication, theeditions indicated were valid. All standards are subject torevision, and parties to agreements based on thisInternational Standard are encouraged to investigate thepossibility of applying the most recent editions of thestandards indicated below. Members of IEC and ISOmaintain registers of currently valid InternationalStandards.

ISO 5-2:1991, Photography - Density measurements - Part2: Geometric conditions for transmission density.

ISO 5-3:1995, Photography - Density measurements - Part3: Spectral conditions.

ISO 5-4:1995, Photography - Density measurements - Part4: Geometric conditions for reflection density.

ISO 12646:____1), Graphic technology - Colour proofingusing a colour display.

CIE Publication No. 13.3, 1995, Method of measuring andspecifying the colour rendering properties of light sources,2nd edition.

CIE Publication No. 15.2, 1986, Colorimetry.

CIE Publication No. 51, 1981, Method for assessing thequality of daylight simulators for colorimetry.

CIE Publication No. 17.4, 1987, International lightingvocabulary.

3 Definitions

For the purposes of this International Standard, thefollowing definitions apply.

3.1 chromaticity: Property of a colour stimulus defined byits chromaticity co-ordinates, or by its dominant orcomplementary wavelength and purity taken together. [CIE Publication No. 17.4, 845-03-34]

3.2 colour rendering index: Measure of the degree towhich the psychophysical colour of an object illuminatedby a test illuminant conforms to that of the same objectilluminated by the reference illuminant, suitable allowancehaving been made for the state of chromatic adaptation.[CIE Publication No. 17.4, 845-02-61]

3.3 correlated colour temperature: Temperature of the

1) To be published.

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Lv

dφv

dA × cosθ × dΩ

Planckian radiator whose perceived colour most closelyresembles that of a given stimulus at the same brightnessand under specified viewing conditions [CIE Publication No. 17.4, 845-03-50]

3.4 flare: Light falling on an image, in an imaging system,which does not emanate from the subject point. Veilingglare is also sometimes referred to as flare. See imageflare, veiling flare, and veiling glare.

3.5 hardcopy: Representation of an image on a substratewhich is self sustaining and reasonably permanent. Seesoftcopy, print, and transparency.

NOTE Examples include prints and transparencies.

3.6 illuminance: (at a point of a surface): Quotient of theluminous flux incident on an element of the surfacecontaining the point by the area of that element [CIE Publication No. 17.4, 845-01-38]

3.7 illuminant: Radiation with a relative spectral powerdistribution defined over the wavelength range thatinfluences object-colour perception.[CIE Publication No. 17.4, 845-03-10] 3.8 image flare: Light from a subject point that is scatteredby the optical system to areas of the image plane other thanthe appropriate image point.

NOTE The distribution of image flare light resulting from anysubject point is specified by the image point spread function. Point spread functions tend to fall off rapidly as the distance fromthe image point is increased, are variable for different image pointlocations and are typically not radially symmetric for image pointssome distance from the optical system axis.

3.9 luminance: (in a given direction, at a given point ofa real or imaginary surface): Quantity defined by theformula:

where dφv is the luminous flux transmitted by anelementary beam passing through the given point andpropagating in the solid angle dΩ containing the givendirection; dA is the area of a section of that beamcontaining the given point; θ is the angle between thenormal to that section and the direction of the beam.[CIE Publication No. 17.4, 845-01-35]

3.10 off-press proof print: Print produced by a methodother than press printing whose purpose is to show theresults of the colour separation process in a way thatclosely simulates the results on a production press.

3.11 on-press proof print: Print produced by pressprinting (production or proof press) whose purpose is toshow the results of the colour separation process in a way

that closely simulates the results on a production press.

3.12 original: The scene or hardcopy from which imageinformation is obtained, using an image capture device, in areproduction process.

NOTE As used in graphic technology, the original is typically aprint or transparency, and the capture device is usually an inputscanner or, occasionally, a process camera. In photography theterm original scene is sometimes used. 3.13 print: A two-dimensional hardcopy form of an imageintended for viewing. In still photography and graphictechnology, the term print is reserved for reflectionhardcopy; a medium designed to be viewed by reflectedlight. See hardcopy, softcopy, transparency.

3.14 relative spectral (power) distribution: Ratio of thespectral (power) distribution of a source or illuminant to afixed reference value which can be an average value, amaximum value, or an arbitrarily chosen value of thisdistribution.

3.15 softcopy: Representation of an image produced usinga device capable of directly representing different digitalimages in succession and in a non-permanent form; themost common example being a monitor. See hardcopy.

3.16 source: A primary emitter of electromagneticradiation.

3.17 surround: The area adjacent to the border of animage which, upon viewing the image, may affect the localstate of adaptation of the eye.

NOTE The surround, which can have a significant effect on theperceived tone and colour reproduction of an image, should not beconfused with any border immediately surrounding the image suchas any unprinted white substrate for reflection copy or theunexposed border present on many transparencies. For a colourmonitor the border will normally be dark grey or black, and hencethe same as the surround. However, when simulating hardcopy itwill be similar to that hardcopy, both in terms of lightness andwidth. 3.18 transparency: A two-dimensional hardcopy form ofan image designed to be viewed by transmitted light. Seehardcopy, softcopy, print.

3.19 transparency illuminator: Apparatus used for backillumination of a transparency.

3.20 veiling flare: The relatively uniform but unwantedirradiation in the image plane of an optical system, causedby the scattering and reflection of a proportion of theradiation which enters the system through its normalentrance aperture. The radiation may be from inside oroutside the field of view of the system.

NOTE Light leaks in an optical system housing can causeadditional unwanted irradiation of the image plane. This

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irradiation may resemble veiling flare. 3.21 veiling glare: Light falling on a radiant imagesurface, such as a back illuminated transparency or monitor,which adds to the luminance of the image. Veiling glarelightens and reduces the apparent contrast of the darkerparts of an image. It differs from veiling flare in that it isused exclusively for the perception of images in which noentrance aperture is defined.

3.22 virtual metamer: A set of spectral radiance factors,not based on physical samples, which provide metamericmatches for specific standard daylight illuminants.

NOTE Virtual metamers are used to test and classify illuminationsources which simulate daylight according to the method providedin CIE Publication 51. This classification is accomplished bycalculating the average of the colour differences obtained for thesemetamers between the illumination source in question and a CIEstandard illuminant. Although it may be possible to constructphysical realizations of some virtual metamers, the fact that theymay not be real allows greater flexibility in their design.

4 Viewing condition requirements

4.1 General requirements

In this clause, the requirements that apply to all of thespecified viewing conditions are stated. The requirementsspecific to each of these viewing conditions are defined in4.2 (Critical comparison), 4.3 (Practical appraisal of prints)and 4.4 (Projection viewing of small transparencies).

NOTE For ease of reference, each viewing condition described inthe International Standard has been given an alpha-numericdesignation. This may be useful in describing or specifyingconditions: e.g. ISO viewing condition P2 as specified in thisInternational Standard.

4.1.1 Viewing apparatus

To comply with this International Standard, the valuesspecified shall be achieved at the surface of viewing. Thespecified spectral power distribution applies to theilluminated surface rather than to the source (or lamp)because the light from the source may be modified byreflecting and transmitting components of the apparatus,and the required spectral power distribution may beobtained from a mixture of light from different sources.

The source, image being viewed, and observer's eyes shallbe positioned to minimise the amount of light specularlyreflected toward the eyes of an observer on or near thenormal to the centre of the viewing surface.

The surround of a print or transparency shall have a

diffusing surface and shall have a CIELAB chroma valueno greater than 2; i.e. shall appear neutral.

4.1.2 Spectral conditions for the reference illuminant

The relative spectral power distribution of the referenceilluminant for both prints and transparencies shall be CIEilluminant D50 as defined in CIE 15.2 (see table 1). Thisrepresents a phase of natural daylight having a correlatedcolour temperature of approximately 5000 K. Thechromaticity coordinates of illuminant D50 are x10 = 0,3478and y10 = 0,3595 in the CIE chromaticity diagram and u'10 =0,2102 and v'10 = 0,4889 in the CIE 1976 UniformChromaticity Scale (UCS) diagram.

NOTE Chromaticity is specified for the CIE 1964 standardcolorimetric observer to ensure compatibility with the methodspecified in CIE No. Publication 51 which is used to define thedegree of compliance of the illumination to the referenceilluminant in 4.2.2.

4.1.3 Colour rendering index

The CIE general colour rendering index of the viewingsurface shall be measured as specified in CIE PublicationNo. 13.3 and shall have a value of 90 or higher. Inaddition, the separate special colour rendering indices forsamples 1 to 8 as specified in CIE Publication No. 13.3shall each have a value of 80 or higher.

4.1.4 Ambient conditions

The visual environment shall be designed to minimiseinterference with the viewing task. It is important toeliminate extraneous conditions that affect the appraisal ofprints or transparencies and an observer should avoidmaking judgements immediately after entering a newillumination environment because it takes a few minutes tovisually adapt to that new environment.

Extraneous light, whether from sources or reflected byobjects and surfaces, shall be baffled from view and fromilluminating the print, transparency, or other image beingevaluated. In addition, no strongly coloured surfaces(including clothing) should be present in the immediateenvironment.

NOTE The presence of strongly coloured objects within theviewing environment is a potential problem because they maycause reflections which cannot easily be baffled and may influenceviewer adaptation. Walls, ceiling, floors, and other surfaces which are in thefield of view shall be baffled or coloured a neutral mattgrey, with a reflectance of 60% or less.

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Table 1 - Relative spectral power of reference illuminant D50.

Wavelength(nm)

Relative power forilluminant D50

Wavelength(nm)

Relative power forilluminant D50

300305310315320325330335340345

0,02 1,03 2,05 4,91 7,78 11,26 14,75 16,35 17,95 19,48

550555560565570575580585590595

102,32101,16100,00 98,87 97,74 98,33 98,92 96,21 93,50 95,59

350 355 360 365 370 375 380 385 390 395

21,01 22,48 23,94 25,45 26,96 25,72 24,49 27,18 29,87 39,59

600605610615620625630635640645

97,69 98,48 99,27 99,16 99,04 97,38 95,72 97,29 98,86 97,26

400405410415420425430435440445

49,31 52,91 56,51 58,27 60,03 58,93 57,82 66,32 74,82 81,04

650655660665670675680685690695

95,67 96,93 98,19100,60103,00101,07 99,13 93,26 87,38 89,49

450455460465470475480485490495

87,25 88,93 90,61 90,99 91,37 93,24 95,11 93,54 91,96 93,84

700705710715720725730735740745

91,60 92,25 92,89 84,87 76,85 81,68 86,51 89,55 92,58 85,40

500505510515520525530535540545

95,72 96,17 96,61 96,87 97,13 99,61102,10101,43100,75101,54

750755760765770775780

78,23 67,96 57,69 70,31 82,92 80,60 78,27

NOTE The wavelength specification has been extended beyond the normal visual range because of the need to considerbrighteners or dyes which may fluoresce.

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It should be noted that it may be easier to minimise theseproblems by using a viewing booth, rather than designingan open area for viewing within a room. Such apparatuscan also make it easier to meet the specification forsurround conditions specified in 4.2.4 and avoid theexcessive flare which may otherwise cause problems ontransparency illuminators. However, even with suchapparatus, adaptation and avoidance of extraneous light stillneed to be carefully considered.

4.1.5 Maintenance

Manufacturers of viewing apparatus shall specify theaverage number of hours during which the apparatus isexpected to remain within specification. The apparatusshould include a time-metering device or some othermechanism for indicating degradation.

However, it is the responsibility of the user, both beforeand beyond this time limit, to undertake measurements asspecified in clause 5 to ensure compliance, unless it can beotherwise demonstrated that the equipment remains withintolerance.

4.2 Conditions for critical comparison (ISOviewing conditions P1 and T1)

4.2.1 Applicability

This subclause specifies viewing conditions for the criticalcomparison between two (or more) copies of an image. The comparison is usually either between the original andits reproduction or between different copies of areproduction such as samples from a press run or multiplephotographic prints. The images being compared may beon the same media (reflective or transmissive), or ondifferent reflective media (including photographic orphotomechanical prints and press proofs or off-pressproofs), or even between transmissive and reflective mediasuch as that pertaining when a transparency is compared toa proof of its printed reproduction. The high illuminationlevels specified permit more critical evaluation of colourand tone gradation in higher density areas which may notbe perceived under most practical viewing conditions.

The condition for viewing a print is specified as conditionP1; that for viewing a transparency directly on anilluminator having a diffusing screen (compared to viewingby projection) is specified as condition T1. The latter willnormally be the case for transparencies larger than 10cm x10cm and in graphic technology is generally the case forsmaller transparencies also.

NOTE In the graphic arts industry the primary viewing application involves comparison, which requires that level P1 beused. However, when it is important that tone reproduction thatwill be perceived under lower levels of illumination is assessed, itis recommended that P1 be supplemented by level P2, or the

expected actual viewing condition. It should be noted that both P1and P2 have the same correlated colour temperature of D50.

4.2.2 Illumination

The illumination at the plane of viewing shall approximatethat of CIE standard illuminant D50. It shall have u'10, v'10chromaticity co-ordinates within the radius of 0,005 fromthat specified in 4.1.2 and a colour rendering index asspecified in 4.1.3. When assessed using the methoddefined in CIE Publication No. 51, but using the virtualmetamers defined for the visible range in table 2, it shallfall within category C and should fall within category B. For condition P1, when assessed using the method definedin CIE Publication No. 51, but using the virtual metamersdefined for the ultraviolet range in table 3, it shall have ametamerism index (MIuv) of less than 4. (See annex B forfurther explanation of these tolerances).

NOTE No specification is provided for the ultraviolet emission ofthe illumination for condition T1. In practice fluorescence is notan issue for photographic transparencies and the diffusing surfaceof the illuminator normally absorbs the majority of any ultravioletemission from the source. The categories to which the equipment conforms at the timeof manufacture shall be displayed on the equipment. Where the ultraviolet metameric index is greater than 2, themanufacturer shall specify whether the contribution tovisible energy from ultraviolet excitation is greater than orless than the contribution of illuminant D50.

4.2.3 Illuminance (P1)

The illuminance shall be 2000 lx ± 500 lx, and should be2000 lx ± 250 lx, at the centre of the illuminated viewingsurface area. Any departures from complete uniformityshall be gradually diminishing from centre to edge. For aviewing area up to 1 metre square, the illuminance at anypoint within the illuminated area shall not be less than 75%of the illuminance measured at the centre of the illuminatedviewing surface area. For larger viewing areas, the lowerlimit shall be 60%.

4.2.4 Surround and backing for reflection viewing (P1)

The surround and backing shall be neutral and matt. Thesurround shall have a luminous reflectance between 10%and 60% with the specific value being selected to beconsistent with practical viewing. For many applications amid-grey of 20% reflectance is very convenient and isrecommended where no other condition is defined. However, whatever value is selected it is importantwhen images are being compared that the surrounds foreach are similar; and as a result the ratio of the surroundluminance shall be 1,0 (± 0,2):1.

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Table 2 - Five sets of spectral reflection radiance factor data providing virtual metamers with the standard data provided in CIE Publication 51. To be used for visible range evaluation for illuminant D50.

Wavelength(nm)

Virtual Metamers, Visual range

Set 1 Set 2 Set 3 Set 4 Set 5

Wavelength(nm)

Virtual Metamers, Visual range

Set 1 Set 2 Set 3 Set 4 Set 5400405410415420425430435440445

0,0290,0280,0270,0260,0240,0240,0240,0250,0250,026

0,0440,0560,0630,0740,0810,0880,0890,0880,0830,081

0,0290,0280,0280,0270,0270,0260,0260,0240,0250,026

0,4030,4030,4030,4030,4020,4010,3980,3930,3870,375

0,1750,1770,1790,1820,1840,1870,1870,1860,1810,178

600605610615620625630635640645

0,4270,4730,5150,5520,5820,6080,6300,6460,6590,671

0,0720,0760,0830,0850,0870,0870,0860,0850,0840,084

0,1020,1030,1040,1040,1040,1030,1030,1040,1040,106

0,2380,2400,2410,2400,2370,2340,2290,2280,2280,236

0,2000,2280,2580,2860,3160,3420,3660,3870,4050,422

450455460465470475480485490495

0,0270,0280,0310,0350,0430,0540,0680,0850,1030,121

0,0760,0710,0660,0590,0520,0480,0450,0420,0390,037

0,0270,0290,0310,0340,0370,0450,0560,0670,0770,086

0,3720,3660,3600,3530,3450,3360,3270,3190,3110,304

0,1740,1700,1650,1600,1560,1510,1480,1430,1410,139

650655660665670675680685690695

0,6830,6950,7080,7220,7360,7510,7660,7810,7940,806

0,0850,0860,0880,0880,0880,0870,0860,0860,0860,087

0,1090,1140,1200,1290,1400,1540,1700,1880,2060,227

0,2450,2640,2870,3200,3580,4030,4490,5020,5520,600

0,4370,4510,4660,4820,5020,5220,5430,5640,5840,603

500505510515520525530535540545

0,1360,1480,1560,1590,1600,1620,1640,1670,1720,177

0,0340,0350,0330,0320,0320,0320,0320,0320,0330,033

0,0920,0950,0970,0950,0920,0900,0890,0880,0860,084

0,2960,2890,2810,2760,2710,2650,2600,2550,2510,248

0,1370,1350,1350,1320,1290,1250,1220,1210,1210,121

700 0,817 0,088 0,250 0,646 0,621

550555560565570575580585590595

0,1820,1890,1960,2090,2260,2480,2750,3090,3450,384

0,0330,0320,0300,0320,0360,0410,0450,0490,0550,063

0,0840,0860,0870,0880,0910,0940,0960,0970,0970,100

0,2460,2450,2440,2430,2410,2390,2360,2340,2340,235

0,1210,1190,1160,1100,1080,1130,1190,1310,1490,174

NOTE This data complements that in CIE Publication 51. The standard data, to which the above provides metamers, may beobtained from that document. The above data are NOT available in CIE Publication 51.

© ISO ISO 3664:1999(E)

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Table 3 - Spectral characteristic of non-fluorescent samples providing virtual metamers with the fluorescent samples provided inCIE publication 51 for illuminant D50.

Wavelength(nm)

Virtual Metamersultraviolet range

1 2 3

Wavelength (nm)

Virtual Metamersultraviolet range

1 2 3400405410415420425430435440445

0,6620,6870,7110,7420,7670,7970,8220,8240,8200,816

0,5050,5890,6680,7230,7640,8050,8380,8450,8430,836

0,2120,2930,4010,5070,6130,7370,8420,8680,8660,857

600605610615620625630635640645

0,8380,8390,8400,8420,8440,8460,8480,8500,8510,851

0,8380,8390,8400,8420,8440,8460,8480,8500,8520,854

0,8380,8390,8400,8420,8440,8460,8480,8500,852 0,854

450455460465470475480485490495

0,8100,8080,8070,8070,8040,8060,8100,8120,8140,816

0,8260,8220,8200,8160,8130,8130,8160,8170,8180,819

0,8450,8450,8430,8370,8300,8280,8270,8260,8260,825

650655660665670675680685690695

0,8520,8520,8520,8530,8530,8530,8530,8530,8530,853

0,8560,8560,8560,8560,8560,8580,8580,8580,8580,858

0,8560,8570,8570,8570,8580,8590,8600,8610,8620,863

500505510515520525530535540545

0,8180,8220,8260,8300,8310,8320,8320,8320,8330,833

0,8210,8250,8290,8310,8330,8330,8330,8330,8340,834

0,8250,8280,8310,8340,8360,8360,8350,8340,8350,835

700 0,853 0,859 0,864

550555560565570575580585590595

0,8340,8350,8350,8350,8360,8360,8360,8370,8370,837

0,8340,8340,8340,8340,8350,8350,8360,8370,8370,837

0,8340,8340,8350,8350,8360,8350,8360,8370,8370,837

NOTE This data complements that in CIE Publication 51. The standard data, towhich the above provides metamers, may be obtained from that document. The abovedata are NOT available in CIE Publication 51.

ISO 3664:1999(E) © ISO

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NOTE 1 A wide range of surround reflectances is allowed in thisInternational Standard so that reflection hardcopy images can beevaluated in conditions which are similar to those used in practice.However, extremely light or dark surrounds are not allowedbecause of their large effect on appearance. Where no practicalcondition can be specified a mid-grey of 20% reflectance shouldbe used.

The surround shall extend beyond the materials beingviewed on all sides by at least 1/3 of their dimension. Where objects are being compared, they may be positionededge to edge. The backing should have a luminousreflectance of 2 to 4% to be consistent with the definition inISO 5-4.

NOTE 2 The above requirement may be met by appropriatefinishing of the viewing surface or by introduction of maskingdevices.

4.2.5 Luminance at the surface of the transparencyilluminator (T1)

The luminance at the centre of the illuminated surface ofthe transparency illuminator shall be 1270 cd/m2 ± 320cd/m2 and should be 1270 cd/m2 ± 160 cd/m2. Anydepartures from uniformity shall be gradually diminishingfrom centre to edge such that the luminance (measurednormal to the surface) at any point within the luminous areais not less than 75% of the luminance measured at thecentre of the image plane.

4.2.6 Transparency illuminator diffusioncharacteristics (T1)

The transparency illuminator surface shall provide diffuselight such that the luminance of the surface measured at any angle between 0° and 45° from the normal shall not be lessthan 90% of the luminance at the same point measurednormal to the surface.

4.2.7 Transparency surround (T1)

The surround shall be at least 50 mm wide on all sides. Itshall appear neutral compared to the source and shall havea luminance that is between 5% and 10% of that of thesurface of the image plane of the illuminator in thedirection of observation. A transparency mounted with anopaque border may be viewed without removing the mount.

NOTE This condition is similar to that specified for directviewing of transparencies in the previous version of thisInternational Standard. However, that version specified an'illuminated' surround when transparencies were compared toprints. The purpose of this surround was to effect a reduction intransparency contrast to facilitate comparison to prints. Unfortunately, this method of contrast reduction significantlyreduces tonal differentiation in the dark tones of the image. Withmodern imaging systems, contrast reduction can be achievedthrough a variety of means that maintain shadow contrast. Theilluminated surround approach could therefore result in amisleading interpretation of transparency shadow detail,particularly for low-key subjects. The dark surround has therefore

been incorporated for all assessment conditions in thisInternational Standard. In practice this condition may be met byusing an opaque black mask; such a mask will appear to have aluminance somewhat above absolute black because of viewingflare and ambient illumination falling on the mask.

4.2.8 Relationship between transparency luminanceand print illuminance (P1 and T1)

For critical comparison between transparencies andreflecting materials, the illuminance at the reflectingmaterial surface shall be that specified in 4.2.3 (i.e. 2000 ±500 lx). The transparency illuminator shall have aluminance as specified in 4.2.5 (i.e. 1270 ± 320 cd/m2). However, the combined tolerances must be such that theratio of the maximum luminance of the transparencyilluminator to the maximum luminance of a perfectlyreflecting and diffusing material, at the plane of thereflecting material, shall be 2 (± 0,2):1. The maximumluminance by reflection from the perfectly reflecting anddiffusing material is equal to the incident illuminancedivided by π.

4.3 Conditions for practical appraisal of prints(including routine inspection). (ISO viewing conditionP2) 4.3.1 Applicability

The specifications in this sub-clause are applicable for theappraisal of tone reproduction of individual images,photographic image inspection or the judgement of prints. They are not appropriate for the simultaneous comparisonof media, where colour matching is the primary concern,such as any comparison between proof and photo-mechanical print, transparency and proof (or print), orbetween different photographic prints and transparencies. The only exception is when comparing a print to a colourmonitor, because of the low luminance level exhibited bycurrent monitors, but such comparisons are outside thescope of this International Standard which only deals withappraisal of images on a monitor in isolation from hardcopy (see 4.5). It should be noted that the spectral power characteristicsspecified for P2 are exactly the same as those specified forcondition P1. Therefore, images that match under theconditions of P1 will match under the conditions of P2. However, the reverse is not necessarily true, particularly ifthere are significant dark tonal areas involved.

Experience has shown that the high levels of illuminationspecified for ISO viewing condition P1 can give amisleading impression of the tone reproduction andcolourfulness of an image which will ultimately be viewedby the consumer in much lower levels of illumination. Images that appear quite acceptable when viewed at thehigher levels of illumination may not appear satisfactorywhen viewed at more typical levels of illumination. Toavoid this problem the illumination level for inspection of

© ISO ISO 3664:1999(E)

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photographic prints is often set arbitrarily while manygraphic technology users take proofs into unknownconditions of lower illumination level to verify that theirtone reproduction will prove acceptable in use. Becauseneither the level or characteristics of the illumination inthese situations are controlled, this practice introducesuncertainties into the process and prevents effectivecommunication.

The viewing conditions specified in this subclause areintended to minimise those problems; the viewingconditions specified are for the appraisal of tonereproduction, for photographic image inspection orjudgement of prints, under illumination levels thatcorrespond to an office, library, or a relatively brightlyilluminated area in a residence. By appraising imagesunder such conditions it is possible to ensure that theyprovide a satisfactory tone reproduction; such a judgementcannot be made unambiguously at the higher level ofillumination specified for condition P1.

NOTE In the graphic arts industry the primary viewing application involves comparison, which requires that level P1 beused. However, when it is important that tone reproduction thatwill be perceived under lower levels of illumination is assessed, itis recommended that P1 be supplemented by level P2, or theexpected actual viewing condition. It should be noted that both P1and P2 have the same correlated colour temperature of D50.

4.3.2 Illumination

The illumination at the surface of viewing shall complywith that described in 4.2.2.

4.3.3 Illuminance

The illuminance at the centre of the viewing surface shallbe 500 lx ± 125 lx. The illumination uniformity shallcomply with that described in 4.2.3.

4.3.4 Surround and backing

The surround and backing shall comply with that describedin 4.2.4 except that a white backing may be used whereappropriate, e.g. for display purposes. The surround andbacking shall be assumed to be in compliance with 4.2.4unless communicated otherwise to others involved in theproduction chain, in which case the reflectance of thesurround and backing shall be specified.

4.4 Conditions for viewing small transparenciesby projection (ISO viewing condition T2)

4.4.1 Applicability

The specifications for the equipment used for viewing aprojected image of a slide on a screen are given in thefollowing subclauses. These conditions are not to beconfused with those normally used for viewing slides in acommercial projector where the magnification is generally

much greater and there is no intent to compare such imageswith reflection prints.

4.4.2 Illumination

The light emitted from the screen with an empty slidemount in the gate shall comply with that described in 4.2.2.

4.4.3 Luminance

The luminance at the screen in the direction of the observershall be 1270 cd/m2 ± 320 cd/m2 when measured with anempty slide mount in the projector.

4.4.4 Uniformity of screen luminance.

Any departure from uniformity of screen luminance shallbe approximately radially symmetrical about the centre ofthe screen, the luminance gradually diminishing from thecentre to the edges of the projected image of the open slidemount. When the screen is viewed at any angle up to 25degrees from the perpendicular to its surface, and at thenormal viewing distance for the equipment, the luminanceof any point within the image of the open slide mount shallbe not less than 75% of that at the centre. The screen onwhich the image is displayed shall exhibit no more speckle,scintillation, or graininess than that exhibited by anuntextured flat, matt, front-projection screen.

4.4.5 Surround

The surround shall comply with that described in 4.2.7.

4.4.6 Stray light and flare

Provisions shall be made for shielding the screen from straylight. The surfaces of the stray light shield(s) facing thescreen shall be matt black.

Stray light and flare shall be such that, when evaluated witha test transparency conforming to 5.2, the luminance at thecentre of the spot image on the screen shall not exceed 1%of the maximum screen luminance for any point in thesurrounding field.

4.4.7 Resolution

The resolving power of the optical system shall be suchthat, when evaluated with a test transparency conforming to5.3, all patterns having a spatial frequency up to 40 linepairs per millimetre shall be resolved at any point in theprojected image.

4.4.8 Distortion

The projection system shall not exhibit noticeable spatialdistortion nor cause noticeable chromatic distortion of theprojected image.

ISO 3664:1999(E) © ISO

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4.5 Conditions for appraisal of images displayedon colour monitors.

4.5.1 Applicability

In order to ensure consistency of assessment of imagesviewed on colour monitors it is important that the viewingconditions in which the monitors are placed are reasonablywell specified. However, it should be noted that thespecifications provided in this International Standard arefor images viewed independently of any form of hardcopy;conditions for direct comparisons between hardcopy andsoftcopy (even where a colour transformation designed toprovide a colour match has been applied) are beyond thescope of this International Standard. Thus, thesespecifications can be seen as being primarily relevant wheresuccessive viewing of hardcopy and softcopy takes place. ISO 12646, Graphic Technology - Colour proofing using acolour monitor, currently at Working Draft level in TC 130,is being produced to provide more detailedrecommendations where direct comparison is required. 4.5.2 Chromaticity

The chromaticity of the white displayed on the colourmonitor should approximate that of D65. It shall have u'10,v'10 chromaticity co-ordinates within the radius of 0.025 ofu'10 = 0.1979 and v'10 = 0.4695 in the CIE 1976 UniformChromaticity Scale (UCS) diagram.

NOTE When viewed under the conditions specified in 4.5 themonitor itself will provide the primary adapting stimulus to theeye. The chromaticity of the white of the monitor is not tooimportant in this situation although many users prefer that thechromaticity of that white be close to that of D65. There is someevidence that, at the low luminance levels obtained with monitors,a chromaticity close to that of D65 provides a better evocation ofwhite and, furthermore, such a chromaticity permits a higher levelof luminance to be achieved with current display technology. However, if the monitor is to be directly compared with prints ortransparencies then the chromaticity of the white of the monitorshould be close to that of the hardcopy to which it is beingcompared. This means that a colour monitor used for such apurpose should have a chromaticity close to illuminant D50. Sucha chromaticity is within the tolerance specified in thisInternational Standard. The specification for comparing colourmonitor to hardcopy is described in greater detail in ISO 12646,Graphic Technology - Colour proofing using a colour monitor.

4.5.3 Monitor luminance The luminance level of the white displayed on the monitorshall be greater than 75 cd/m2 and should be greater than100 cd/m2.

NOTE With current display technology the level of luminancethat may be achieved depends upon the chromaticity of the whitepoint of the monitor. As the correllated colour temperature isincreased the level of screen luminance that may be achievedbecomes higher. A level of at least 100 cd/m2 is recommended forthis application but it is accepted that this may be difficult toachieve on some monitors, particularly for older or lower cost

models, or where the monitor white point is set to the chromaticityof illuminant D50.

4.5.4 Ambient illumination When measured at the face of the monitor, with a cosinecorrected photometer and with the monitor switched off,the level of ambient illumination shall be less than, or equalto, 64 lx and should be less than, or equal to, 32 lx. Theselimits must also be achieved when measured in any planebetween the monitor and the observer. The correllatedcolour temperature of the ambient illumination shall be lessthan or equal to that of the monitor white point.

NOTE The level of ambient illumination needs to be significantlylower than the luminance level of the monitor white point. This ispartly to ensure that the observer is reasonably adapted to themonitor but primarily to ensure that the full contrast range of themonitor is not significantly reduced by the effects of veiling glare. It is for these reasons that the level of ambient illumination needsto be less than 64 lx and, preferably, much lower. This isparticularly significant where low luminance monitors areemployed. If the level of ambient illumination approaches thehigher level specified in this clause, the correllated colourtemperature should be approximately the same as the white pointof the monitor, in order to minimise chromatic adaptationcomplications.

4.5.5 Surround condition The area immediately surrounding the displayed image andits border shall be neutral, preferably dark grey or black tominimise flare, and of approximately the same chromaticityas the white point of the monitor. The luminance of theborder should be 20% of the white point luminance, or less,and preferably 3% of the white point luminance, or less.

NOTE When the monitor is being used to visualise images whichwill be reproduced as hardcopy the recommended lightness of anyborder displayed around the image will depend upon thecomparison. In general, for comparison to prints, which may wellbe reproduced with a white border consisting of unprintedsubstrate, the border of the image should be light to simulate thissubstrate; for comparison to transparencies it should be dark. However, it is generally preferable that any such border be nomore than 1 to 2 cm wide, even if it would normally be wider onthe hardcopy reproduction.

4.5.6 Environmental conditions The monitor shall be situated so there are no stronglycoloured areas (including clothing) directly in the field ofview or which may cause reflections in the monitor screen. Ideally all walls, floors and furniture in the field of viewshould be grey and free of any posters, notices, pictures,wording or any other object which may affect the vision ofthe viewer.

4.5.7 Glare All sources of glare should be avoided since theysignificantly degrade the quality of the image. The monitor

© ISO ISO 3664:1999(E)

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shall be situated so that no illumination sources such asunshielded lamps or windows are directly in the field ofview or are causing discernable reflections from the surfaceof the monitor.

5. Test methods

5.1 Spectral measurements

To determine the colour rendering index of theillumination, as defined in 4.1.3, and the chromaticity andmetameric index as defined in 4.2.2, it is necessary tomeasure the spectral power distribution of the illumination. This requires measurement over the range 300 to 730 nmfor conditions P1 and P2 and over the range 380 to 730 nmfor conditions T1 and T2. The bandpass of the measuringinstrument (spectroradiometer) shall be 5 nm or narrower. The sampling interval shall not be greater than thebandpass.

NOTE - As in any measurement process, the measuring equipmentmust be regularly calibrated. In this case the calibration of thespectroradiometer must include assessment of stray light, linearity,wavelength accuracy and spectral power accuracy.

5.2 Illuminance and luminance

All illuminance or luminance measurements shall be madewith a photometer having the spectral responsivity of theCIE standard photopic photometric observer, V(λ), andmeasuring an area having a diameter no greater than 1/20 ofthe shortest linear dimension of the illuminated surfacearea. For illuminance measurements the photometer shallbe cosine-corrected.

For projection viewing apparatus, stray light and flare shallbe evaluated with the aid of a test transparency that is clear,

neutral and transparent everywhere except for an opaquecentral circular spot producing an image having a diameter1/10 the smallest linear dimension of the projected openingin an open slide mount. Measurement shall be madeperpendicular, within 7 degrees, to the spot and 35 cm fromthe screen.

NOTE The measurement should be made over an area not greaterthan 30% of the opaque (black) spot diameter, approximatelycentred on the centre of the spot.

All measurements shall be conducted in the presence ofenvironmental illumination that would normally exist whenthe apparatus is used.

5.3 Resolution assessment for projection viewingapparatus

A test target shall be used containing square waveresolution patterns varying in spatial frequency and withtwo mutually perpendicular orientations. The range offrequencies included should be at least 20 to 60 line pairsper millimetre and should include 40 line pairs permillimetre. The dark bars of the test patterns shall have an ISO visual diffuse transmission density, as defined in ISO5-2 and 5-3, at least 2,0 higher than the density of thetransparent background. A pattern having a given spatialfrequency shall be considered resolved if the sets of lines ofthe projected image of that pattern, oriented both radially tothe centre of the field and tangentially to a circle about thecentre, are clear enough to be counted with reasonableconfidence. The concept of "reasonable confidence" isintended to indicate a level of confidence that is somewherebetween complete confidence and no confidence at all. Allresolution evaluations shall be made with the optical systemset at the same focus. A low-power magnifying glass maybe used.

ISO 3664:1999(E) © ISO

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Annex A (Informative)

Summary of ISO viewing conditions specified in ISO 3664

This table is deemed to be informative since it is included for convenience and introduces no new specifications. It simplysummarises the main normative requirements specified throughout this International Standard.

ISO Viewingcondition

ReferenceIlluminant andchromaticitytolerance (1)

Illuminance/Luminance

Colour Renderingindex (according to

CIE 13.3)

Metamerismindex (according

to CIE 51)

Illuminationuniformity(min:max)

Surroundluminous

reflectance/luminance/

illuminance Critical comparison

Prints (P1)

Transparencies(Direct viewing)

(T1)

illuminant D50(0,005)

illuminantD50

(0,005)

2000 lx ± 500 lx(should be ± 250

lx)

1270 ± 320cd/m2 (should be

± 160 cd/ m2)

(2)

General index: 90Special indices for

samples 1 to 8:80

General index: 90Special indices for

samples 1 to 8:80

Visual: C orbetter and

should be B orbetter

UV: <4

Visual: C orbetter and

should be B orbetter

For surfaces up to1m x 1m 0.75

For surfacesgreater than 1m x

1m 0.6

0,75

<60% (neutraland matt)

5 - 10% of theluminance level

(neutral andextend at least50mm on all

sides)

Practical appraisalof prints (P2)

illuminant D50(0,005)

500 ± 125 lx General index: 90Special indices for

samples 1 to 8:80

Visual: C orbetter and

should be B orbetter

UV: <4

0,75 <60% (neutraland matt)

TransparenciesProjection viewing

(T2)

illuminant D50(0,005)

1270 ± 320 cd/m2

General index: 90Special indices for

samples 1 to 8:80

Visual: C orbetter and

should be B orbetter

0,75 5 - 10% of theluminance level

(neutral andextend at least50mm on all

sides) Colour Monitors chromaticity of

illuminant D65(0,025)

> 75 cd/ m2

(should be> 100 cd/ m2)

Not applicable Not applicable Not applicable Neutral, anddark grey or

black (3)

Notes1 This specifies the spectral power distribution of the reference illuminant, except for colour monitors in which case it specifies the chromaticity of the

white point of the monitor. Permitted tolerances in chromaticity, from that of the reference illuminant, are given in parentheses. These are specified atthe plane of viewing, according to the 1976 u'10, v'10 UCS system.

2 When comparing a transparency to a print the ratio of the luminance of the transparency illuminator to the equivalent illuminance of the print viewingsurface shall be 2 (± 0,2):1.

3 The ambient illumination for colour monitors should be less than or equal to 32 lx and shall be less than or equal to 64 lx.

© ISO ISO 3664:1999(E)

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Annex B (informative)

Experimental data leading to selection of metamerism indices

And reference illuminant for this International Standard

B.1 Introduction

In order to establish a suitable reference illuminant for thisInternational Standard, as well as determining toleranceswhich are both acceptable and practical, a good deal ofexperimental work was carried out. The objective of thework was threefold; to determine which referenceilluminant would be most appropriate for specifyingviewing conditions for the evaluation of hardcopy imagesfor both Graphic Technology and Photography; to definethe most appropriate measures of acceptability between thisreference illuminant and real sources (together with anyother components of the viewing environment); and toestablish suitable tolerances for these measures.

Since it was considered important that any tolerancesspecified were not unreasonably stringent, a small numberof practical sources were studied rather extensively. Subsequently the work was extended to verify that areasonably large number of existing apparatus, in use inindustry, were not significantly deviant from thesetolerances, although it would not be a concern if many weresomewhat outside. Since much of this apparatus had beenin use for some time this would not be consideredunacceptable. It is inevitable that in such a situation manyviewing environments would be due to be updated, or needto have their sources replaced. It would be hoped, andexpected, that such a situation would result in apparatusthat is more consistent to one another than with olderequipment.

B.2 Reference illuminant and acceptabilitytolerances

In the 1974 version of this International Standard thereference illuminant had been specified as D50 and theacceptability tolerances had been specified by a limit on thepermitted chromaticity deviation from D50, together withcolour rendering indices as defined by CIE publication13.3. However, in the intervening years various workershad expressed reservations about this specification for quitedifferent reasons. Many practical users had been heard tocomment on the wide variation in colour of the illuminationthey perceived between apparatus that were assumed tomeet the 1974 specification; because of this it wasconsidered that the specification of the tolerance in

chromaticity was therefore in need of some reduction. Various research workers had noted that the mediaencountered in the industries for which this InternationalStandard was appropriate were far more metameric thanhad previously been the case. The colour rendering index(which simply measures the difference in colour for eightMunsell samples as they would appear under both test andreference illuminants) provided no indication that differentapparatus conforming to this specification would rendersuch practical metamers in a similar way. Furthermore, ithad been suggested that since most viewing apparatus used fluorescent tubes for simulating the reference illuminantthey could not be spectrally similar to D50; supporters ofthis position concluded that a new reference illuminant wastherefore more appropriate because of the metamerismdescribed earlier.

At the time this revision of the International Standard wasstarted, proposals had already emanated from the USA(though not finally agreed nationally) to reduce thetolerance on chromaticity, replace the reference source withF8 (a spectral power distribution defined by CIE as‘typical’ of fluorescent tubes with a chromaticity the sameas D50) and add a ‘band method’ to reduce the incidence ofmetamerism.

The ISO joint working group felt that there was substantialjustification for considering these proposals. However, itwas agreed that it was desirable to undertake someexperimental work to verify their acceptability. It was alsonoted that the ‘band method’ was not supported by CIEwho had, in 1981, published a procedure (CIE publication51) for verifying the acceptability of daylight simulators. This provides a set of theoretical metamers for each of thethree reference illuminants D55, D65 and D75 and specifies aprocedure for calculating a Metamerism Index.

B.3 Metamerism index

The CIE method specifies eight theoretical metamers; thatis, the relative spectral power for colours which match(thereby giving a colour difference of 0) for a specifiedilluminant and observer. Five of these are metameric in thevisible region of the spectrum, the other three arefluorescent samples which are metameric with respect tothe ultra-violet radiation emitted by the source. The

ISO 3664:1999(E) © ISO

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Sn(λ)

100 S(λ)

700

400S(λ) y10 (λ) ∆(λ)

specified procedure requires measurement of the spectralpower distribution of the test source and calculation of thetristimulus values for these metamers for both the test andreference source, normalised by area according to equation1, so that the assessment is independent of the absolutelevel of illumination.

(1)

If the test source has a different spectral power distributionto the reference illuminant the metamers are likely to fail tomatch under the former; thereby giving rise to a colourdifference between each pair of metamers. The magnitudeof this difference will depend upon the differences betweenthe test and reference illuminants’ relative energies. TheCIE procedure then requires that the differences beaveraged to produce two indices, one for the visible regionand one for the ultra-violet. A category scale is definedwhich ranks the test source, covering the range from A(best) to E (worst).

Unfortunately, CIE Publication 51 does not providemetamers for D50. To enable this method to be used by theJoint task Force, the necessary metamers were provided byMcCamy. He used the same reference data as that in CIEPublication 51 and calculated them from the metamersprovided in that publication by extrapolation. The resultantmetamers, defined for illuminant D50 and the CIE 1964Supplementary Standard Colorimetric Observer, are givenin tables 2 and 3 of this document. It is anticipated thatthey will be accepted by CIE as an additional data set forCIE Publication 51 in due course. The full details of thederivation can be found in Color Research and.Application, Volume 21, Number 3, pp 236-237, June1996.

B.4 Experimental work

The experimental work undertaken initially was dividedinto 3 stages. The first was to evaluate whether there wasany advantage in using F8 as the reference illuminant. Thesecond was to determine whether the metamers proposed by extrapolation from those published in the CIE documentprovided reasonable correlation with the differencesanticipated for practical metamers found in practice. Thethird was to determine the effect of the standardcolorimetric observer selected. (The CIE 51 method usesthe 1964 Supplementary Standard Colorimetric Observer,whereas it is common industry practice to use the 1931Standard Colorimetric Observer).

In order to achieve this two additional sets of metamerswere calculated, derived from practical samples.

For the first set of metamers, the reference consisted of thespectral reflectance data listed in ANSI CGATS TR001-

1995 for the 928 colours defined in ISO 12642. (These areeffectively the colours produced when the data set isprinted according to the US Specifications for Web OffsetPublications - SWOP). The metamers to the SWOPpatches were calculated for the cyan, magenta, and yellowdyes of a continuous tone dye diffusion proofing printer,using a tristimulus matching algorithm. Three referenceilluminant and observer combinations were used to derivethree sets of metameric matches (D50 /CIE 1964Supplementary Standard Colorimetric Observer, F8 /CIE1964 Supplementary Standard Colorimetric Observer, andD50 /CIE 1931 Standard Colorimetric Observer). Themetamers calculated were defined such that they had acolour difference (∆E*a b) of less than 0,02.

The reference for the second set of metamers consisted ofspectral reflectances for 8 colour patches printed withoffset inks according to ISO 12647-2 (paper type 1). Themetamers of these printed patches were calculated using aspecial matching algorithm with 8 sets of cyan, magentaand yellow colorants belonging to the imaging systemsshown in table B.1.

Six commercial fluorescent lamps (labelled A to F), allnominally D50 simulators, were measured and evaluatedusing all three sets of metamers described above (the setprovided by McCamy, the set based on the SWOP printing,and the set based on ISO 12647-2). Tables B.2 and B.3summarise the results of these calculations. The deviationin chromaticity of the lamps from D50, calculated using theCIE 1976 Uniform Chromaticity Scale, is included in thefirst column of each table It had been proposed when thisrevision of this International Standard was started that thetolerance be defined as a circle of radius 0,005 centred onthe chromaticity of D50. It should be noted that lamps Cand F do not meet the chromaticity recommendation.

Each table contains the CIE visible range metamerismindex (MIVIS) based on the McCamy metamers (the averagecolour difference for the 5 metamers). As described in theCIE publication, the MIVIS is converted to a category scale,in which a ‘C’ has a value ranging from 0,5 to 1,0, and a‘D’ ranging from 1,0 to 2,0. These results show that mostlamps fell within the category ‘C’ (proposed as theminimum in this specification), but that two of them wereoutside this criteria.

Also shown in tables B.2 and B.3 are the values for theaverage and maximum ∆E*a b encountered with eachpractical data set. Comparing the metameric index MIVISwith the metameric differences obtained with differentimaging systems, it may be seen that the index values arefalling between the average and the maximum colourdifferences. The “artificial” metameric pairs used for theindex calculation may therefore be regarded as validindicators of metameric differences based on thecomparison with differences occurring with real imagingsystems.

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The testing with the metamers based on the 928 SWOPsamples was also extended to derive two additional sets ofmetameric matches for F8 /CIE 1964 SupplementaryStandard Colorimetric Observer, and D50 /CIE 1931Standard Colorimetric Observer. Again the metamerscalculated were defined such that they had a colourdifference (∆E*a b) of less than 0,02. Tables B.4 and B.5show these data. (It should be noted that although theMIVIS and category data are shown in each table, thesevalues are only calculated for D50 and the CIE 1964Supplementary Standard Colorimetric Observer as definedin CIE Publication 51.)

The average ∆E*a b statistics were similar whether D50 or F8were taken as the reference (with the CIE 1964Supplementary Standard Colorimetric Observer) and therewas no consistent change in deviation between the two setsof results. The magnitude of the values was generallylarger for the 1931 Standard Colorimetric Observer, butthis was not unexpected; it has been reported before.

However, based on the 928 SWOP samples, the relativeranking between the lamps, was very similar regardless ofwhich illuminant or observer was taken as the reference(see tables B.3, B.4, and B,5). The worst three were lampsF > A > C, while D, B, and E’s ranking varied slightly. This ranking was the same when the proposed CIE D50metamers were used. For this reason it was decided that weshould continue to specify D50 as the reference illuminant inthis International Standard to enable consistency withexisting products and use the CIE method with themetamers proposed by McCamy. It was accepted that theexperimental data (despite the fact that it is fairly extensiveof itself) was fairly minimal in terms of the possiblevariations which may be encountered in practice and moredata would always be welcome. When the data for the 8metamers based on the 8 colour patches were subsequentlyprovided, it confirmed the view, as described earlier, thatthe CIE method (based on illuminant D50) was probablyrobust enough for application in this International Standard.

On the basis of the proposals made in this InternationalStandard only 3 of the lamps tested would be acceptable. C would fail because of its chromaticity, A because of itsmetamerism index and F on both counts. However, it isprobable that the first of these lamps could easily be madeacceptable by suitable design of the apparatus. This ‘pass’rate was considered acceptable in the interest of minimisingthe difference between products, as requested, by many

users and led us to confirm this specification. Support wasgiven to this when we were supplied with data showing anumber of Japanese lamps, not tested in this study, allconformed to the specifications proposed.

As a final validation of the proposals made in thisspecification, 61 viewing apparatuses were measured insitu. It was found that only 11% fully met the chromaticity,colour rendering and metamerism index criteria. Another17% met the latter two but failed slightly on chromaticity. 13% met the chromaticity and colour renderingspecification, but failed slightly on metamerism index. 7%met the metamerism index and chromaticity criteria butfailed slightly on colour rendering. The remaining 52%failed on at least 2 criteria but in many cases the deviationsin both cases were relatively small.

Given that the maintenance schedule on these units wasunknown, it was this data that gave us the final confidencethat the specification proposed was about right. Given therequirement by the industry that deviations between theviewing apparatuses be reduced, simply producing aspecification that includes all current apparatus that meetsthe 1974 specification would be rather pointless. Byreducing the chromaticity tolerance from that required in1974, and adding the metamerism index, the industrialrequirements should, in part at least, be met. Whilst it is aguess, it seems likely that the number of these existing unitsfully meeting the specification could be significantlyincreased by cleaning and replacement of old tubes. Otherscould be brought into line by suitable modification of theapparatus itself (reflectors, diffusers, etc). But mostimportantly, knowing that lamps exist with satisfactoryproperties, it is clear that equipment vendors can beexpected to modify their designs to meet the newspecification. The only question must be whether the lampsamples measured are taken from batches with large orsmall distributions of the various criteria but we have noreason to believe that those selected are in any wayuntypical.

Thus, users who wish to improve communication byreducing the deviation between their units and others canchoose to try and modify existing apparatus or purchasenew. This must be the compromise between userrequirements and practical production so important in anInternational Standard. Of course, at the next revision wemay find that production has improved such that we canreduce the specification yet further; only time will tell!

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Table B.1 Imaging systems used to assess the metamerism of light sources

Process DeviceElectrophotography (dry toner) Canon CLC 800

Xerox DocuColor 40Electrophotography (liquid toner) Indigo E-Print 1000

Ink jet (continuous) Scitex Iris

Ink jet (solid) HP Design Jet 750

Thermal transfer (dye sublimation) Tektronix Phaser 440

Silver halide Fuji Pictrography 3000

Photomechanical Fuji Color-Art

Table B.2. Matched for D50, CIE 1931 Standard Colorimetric Observer, 8 imaging systems, 8 patches.

Lamp(chromaticitydifference)

CIE51 MIVIS (1) CIE51 CategoryMetameric sample difference (2)

Average ∆E*a b Maximum ∆E*a b

A (0,003) 115 D 50 145B (0,005) 88 C 42 91C (0,008) 97 C 59 133D (0,001) 73 C 55 134E (0,002) 89 C 48 120F (0,007) 242 E 160 387

F8 63 C 29 87Note1 Values refer to the CIE 1964 Supplementary Standard Colorimetric Observer2 Values refer to the CIE 1931 Standard Colorimetric Observer

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Table B.3. Matched for D50, CIE 1964 Supplementary Standard Colorimetric Observer, 928 patches

Lamp(chromaticitydifference)

CIE51 MIVIS (1) CIE51 CategoryMetameric sample difference (1)

Average ∆E*a b Maximum ∆E*a b

A (0,003) 115 D 94 175B (0,005) 88 C 72 166C (0,008) 97 C 74 169D (0,001) 73 C 65 181E (0,002) 89 C 59 167F (0,007) 242 E 120 306

F8 63 C 34 167Note1 Values refer to the CIE 1964 Supplementary Standard Colorimetric Observer

Table B.4. Matched for F8, CIE 1964 Supplementary Standard Colorimetric Observer, 928 patches

Lamp(chromaticitydifference)

CIE51 MIVIS (1) CIE51 CategoryMetameric sample difference (1)

Average ∆E*a b Maximum ∆E*a b

A 115 D 97 173B 88 C 79 166C 97 C 87 210D 73 C 60 181E 89 C 58 167F 242 E 121 283

D50 33 178Note1 Values refer to the CIE 1964 Supplementary Standard Colorimetric Observer

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Table B.5 Matched for D50, CIE 1931 Standard Colorimetric Observer, 928 patches

Lamp(chromaticitydifference)

CIE51 MIVIS (1) CIE51 CategoryMetameric sample difference (2)

Average ∆E*a b Maximum ∆E*a b

A 115 D 119 291B 88 C 84 188C 97 C 94 202D 73 C 76 226E 89 C 81 258F 242 E 138 303F8 63 C 43 188

Note1 Values refer to the CIE 1994 Supplementary Standard Colorimetric Observer2 Values refer to the CIE 1931 Standard Colorimetric Observer

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Annex C (informative)

Guidelines for judging and exhibiting photographs

C.1 Introduction

The subjective impression produced by photographs thatare judged for contests or juried exhibitions is of particularimportance. It follows that photographs intended for suchpurposes should be optimised for a particular, knownviewing condition. The viewing conditions specified in thisInternational Standard are appropriate for this application,but other considerations have led to the use of somewhatwider tolerances in certain directions, and illuminationlevels that at first glance might seem inappropriate.

The preferred illumination levels for judging and exhibitionare conditions P1, T1 and T2. The higher illuminationlevel is chosen to allow maximum utilisation of the densityrange and colourfulness capabilities of output media,particularly for judging. Prints and transparenciesoptimised for these illumination levels will appear a bitdark when viewed under typical indoor illumination, butphotographers produce the darker images specifically forcontests. The feeling is that the increase in "impact"resulting from the larger dynamic range is more importantthan having the prints look perfect under typical indoorillumination. It is highly desirable, therefore, thatillumination levels similar to those specified for ISOviewing conditions P1, T1 and T2 be adhered to forjudging. For exhibition, it may be impractical to achievethe high levels of illumination required, and lower levelsare acceptable.

Another practical consideration concerns the spectral powerdistribution of the illumination The human visual system isexcellent at adapting to the correlated colour temperature ofillumination sources which approximate a Planckianradiator between 2800 K and 6500 K. Along this locus,typical photographic materials exhibit larger perceptualshifts due to sources failing to approximate a Planckianradiator than from changes in correlated colourtemperature. This is particularly true for sources whichemit sharp spectral lines. A fluorescent source may haveexactly the same chromaticity as a particular Planckiansource, but a photographic material may appearsubstantially different under the two sources.

The purpose of the metamerism and colour rendering indexrequirements in this International Standard is to minimisethe likelihood of substantial changes in appearance due todifferent source spectral power distributions. If the ISOviewing condition recommendations are adhered to,

metamerism should be minimised and colour renderingshould be appropriate. Unfortunately, many smallphotographic associations do not have access to lightingequipment that meets these strict requirements. Thequestion then becomes: if departure from this InternationalStandard is forced by practical considerations, in whichdirections can tolerances be extended with minimal impact?

In the photographic community, experience has indicatedthat the smallest effect on the perception of photographsresults from allowing the correlated colour temperature todecrease. Tungsten sources can be filtered to produceexcellent 5000 K illumination, but there is a financial costdue to the filters required, and the fact that more powerfulsources are needed to produce the same illumination level. Fluorescent sources designed to meet the metamerism andcolour rendering index requirements of this InternationalStandard are also expensive, and fluorescent sources tendto be diffuse. The more directional illumination producedby tungsten sources can be positioned to minimise surfacereflections, thereby affording the best view of high dynamicrange prints.

The result of all these considerations is that the sourcespreferred for judging photographic prints are moderatelydirectional tungsten or filtered tungsten sources withcorrelated colour temperatures between 2800 K and5000 K. The use of filtered tungsten minimisesmetamerism and colour rendering problems, anddifferences in correlated colour temperature are dealt withby the adaptation of the human visual system. Tungstenand filtered tungsten sources are also used for projectedtransparencies. Fluorescent sources which meet therequirements of this International Standard are preferredfor the direct viewing of transparencies, where diffuseillumination is desirable.

C.2 Recommendations

Table C-1 summarises the recommendations for the judgingand exhibition of photographs. These recommendationsassume that comparisons will be made only of similarmedia under the same illumination, and that observers areadapted to the illumination. If a non-standard correlatedcolour temperature or illumination level is used, it shouldremain constant for all evaluations conducted. If prints andtransparencies are to be compared, non-standardillumination should be avoided to the greatest extentpossible. Any comparisons that involve different effective

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correlated colour temperatures or illumination levels shouldbe successive, and observers should be given sufficienttime to adapt to the illumination before being allowed toview the photograph. Also, the correlated colour

temperature of any ambient illumination present whentransparencies are viewed should be less than or equal tothe correlated colour temperature of the viewingillumination.

Table C-1. Recommendations for viewing conditions for judging and exhibiting photographs.

Material Recommended viewing conditions

Judging:

photographic prints ISO condition P1 with 0°/45° viewing geometry, but the correlated colour temperature can beas low as 2800 K if tungsten sources are used 1.

transparencies(direct viewing)

ISO condition T1 (diffuse illumination).

transparencies(projection viewing)

ISO condition T2, but the correlated colour temperature can be as low as 2800 K if tungstensources are used1.

Exhibition

photographic prints ISO condition P1 with 0°/45° viewing geometry, but the correlated colour temperature can beas low as 3200 K if tungsten sources are used1, and the illuminance level can be as low as 375lx.

transparencies(direct viewing)

ISO condition T1 (diffuse illumination), but the correlated colour temperature can be as low as2800 K if tungsten sources are used1, and the luminance level can be as low as 240 cd/ m2, or1000 times the veiling glare luminance, whichever is greater2.

transparencies(projection viewing)

ISO condition T2, but the correlated colour temperature can be as low as (projection viewing)2800 K if tungsten sources are used1, and the luminance level can be as low as 40 cd/ m2, or1000 times the veiling glare luminance, whichever is greater3.

1) With sources at correlated colour temperatures other than 5000 K, the metamerism and colour rendering index qualifications describedin this International Standard are not applicable. In such cases, the user must rely on the similarity of the tungsten source spectralpower distribution to that of a theoretical Planckian radiator. If the spectral power distributions are sufficiently similar, thechromaticities, metameric differences, and colour rendering will also be similar.

2) The veiling glare luminance in a typical direct viewing situation will be approximately 0,0016 times the stray light illuminanceincident on the illuminator surface.

3) The veiling glare luminance in a typical projection viewing situation will be approximately 0,3 times the stray light illuminanceincident on the projection screen.